N, M = map(int, input().split())

ans = 0
for bit in range(30):
    if bit % 2 == 0:
        if M >> bit & 1 == 1:
            if N >> bit & 1 == 0:
                ans = -1
                break
            else:
                ans |= (1<<bit)
    else:
        if M >> bit & 1 == 0 and N >> bit & 1 == 1:
            ans = -1
            break
        elif M >> bit & 1 == 1 and N >> bit & 1 == 0:
            ans |= (1<<bit)

print(ans)